As is known, in the production of sugar a sludge is produced which in addition to lime (CaCO.sub.3), sucrose and albumen, contains small quantities of other constituents and has a varying moisture content. Previously, this lime sludge has often been used as fertilizer; in recent years it has usually been simply deposited somewhere, which involves more or less considerable expense. This is not desirable from the point of view of avoiding pollution of the countryside and general environment.
Various attempts have already been made for recovering lime from lime-containing sludge in sugar production works by burning.
For example in the United States of America, some time ago a plant was built with a multi-tier furnace comprising eleven tiers or stages and fired with additional fuel. This plant, however, could achieve only a yield of 65%, since because of the way the plant operated a considerable proportion of the lime was carried away by the discharged gases. Preventing this would have required an expensive installation for extracting dust from the waste gases. According to reports, the operating behaviour of the said plant is not satisfactory.
Again in the United States of America it has already been attempted to calcine lime sludge with a very long and complicated shape rotary tubular furnace. This kind of apparatus can in fact obtain better operating results than that previously mentioned, but the rotary tubular furnace itself and also its operation is so expensive as to prevent it being used in sugar factories even for reasons of cost alone. Furthermore, the furnace does not operate in a troublefree manner, so that there are frequent interruptions in working, more particularly owing to material caking-on in the form of rings in certain zones of the furnace. But with a furnace length of more than 90m these accumulations can be removed only if the furnace is stopped and cooled, and this takes several days.
A primary object of the present invention resides in provision of a unique process for the production of a powdered, surface active, agglomeratable, calcined material from a raw material using combustion gases and exhaust gases to preheat and react the raw materials under temperature and time controlled reacting conditions. A further object resides in isolating the produced powder from reaction gases produced during the reaction.
The invention has a more particular object to make a sensible use of the waste product produced in sugar production, e.g., the lime-containing sludge, and to develop for this purpose a rational process which consists substantially in preparing the lime-containing sludge, using the calorific value of its organic impurities, to obtain CaO again as far as possible, which can then be re-used in sugar production in a particularly simple and economical manner.
In sugar production, lime milk is required and for this purpose the CaO is hydrated and dissolved in water. Therefor, a further object is to produce the CaO in such a form that it is capable of being hydrated and dissolved in water in as easy and rapid a manner as possible. This can be achieved by so controlling the process for the production of CaO that a highly active CaO powder is obtained which reacts chemically and physically very quickly.
Behind the aforesaid objects of the invention there is also the objective of providing a method which enables the construction of and use of novel installations involving low constructional outlay and having high operating flexibility. Apparatus expense is very important more particularly in sugar production since, as is known, sugar producing apparatus are in use only for three to four months in the year, so that the construction outlay, that is to say investment and other capital costs, have about three to four times as much weight as in other branches of industry where the plant can be used uniformly over the entire year. The flexibility of the process, i.e. the possibility of adapting it to various operating conditions, is important since on the one hand the organic constituents in the lime sludge fluctuate considerably, so that different quantities of heat are produced which have to be compensated for, and on the other hand the installation in question must be capable, for example at the beginning of a sugar campaign or under other operating conditions, of dealing if appropriate with even pure CaCO.sub.3 or mixtures of CaCO.sub.3 and sugar lime sludge.
To solve the aforesaid problems and to obviate the disadvantages of known processes and installations or apparatus for the recovery of lime from lime-containing waste sludges for example in the sugar industry or the like, according to the invention it is proposed to use a process wherein, in detail, the following procedure is used: preheated and predried lime-containing sludge and preheated combustion air and, if necessary, additional fuel are introduced into a chamber and mixed thoroughly with one another, and the combustible constituent are burned. These materials flow through the chamber at least partly in an upward direction from below, the lime particles being subjected to a heat treatment during which they are kept in a state of suspension. Here it is an important objective of this invention that the lime particles are heated as quickly as possible, and in fact with a speed of heating which is above about 400.degree. C/sec; the upwardly directed flow of the air stream carrying the particles has such a speed that the average time of residence of the lime particles in the combustion and reaction zone in the chamber amounts to less than one second. The thoroughly desirable effect is obtained that the time of residence of smaller particles in the combustion and reaction zone is shorter and the period of residence of the large particles is somewhat longer, so that on the one hand a complete calcining of the lime particles is achieved and on the other hand over-heating is avoided. Those particles which are too large or too heavy to be carried in the air flow fall downwards and are collected there and removed. These conditions can be controlled in a simple manner by selecting the speed of the combustion air flow passing upwards in the chamber. The consequence of this is again a very uniform calcining of all lime particles. But in addition, with a view to maintaining the desired temperature in the heat treatment chamber, it is necessary to control introduction of the addition fuel in such a manner that on the one hand no overheating occurs and on the other hand no inadequate temperatures occur. This means that owing to the usually fluctuating calorific value of the combustible substances in the lime sludge the introduction of any additional fuel which may be necessary is to be regulated in accordance with the calorific value of the sludge. A further feature of this process consists in that the air ratio relative to the theoretical quantity of combustion air is between about 1.4 and about 2.5, but preferably between about 1.6 and about 2.0; this means in practice that the quantity of combustion air is regulated so as to maintain the aforesaid air ratio. By means of all the measures described hereinbefore the result is achieved that the CaO particle, independently of the size of the particles, are heated approximately all to the same predetermined combustion temperature which is kept approximately constant, and are heated in a very short time.
If the lime-containing sludge is treated in the aforesaid manner there is obtained a graded, surface-active lime powder of specific particle size, which is carried by a gas flow from which the powder has then to be separated and precipitated. But this is possible in an economical manner with relatively small outlay on apparatus only if the powder particles carried by the gas flow have a sufficiently high surface activity, since in that case they agglomerate to form secondary particles and in this way allow easy separation from the exhaust gas. If there were no activation of the dust particles and thus no formation of secondary particles, the extraction of powder from the outgoing gases would be so expensive as to prejudice the economic aspect of the process.
As a further object, commensurate with the foregoing, in order to prevent any danger of recombination when cooling the finished product in the presence of CO.sub.2 -containing exhaust gases, it is advantageous in the region of the separating zone to use an air flow to shield the separated hot CaO from the exhaust gas and to mix it with an air flow whereby it is transported further. By means of the air flow the exhaust gas containing CO.sub.2 is substantially displaced and separated from the lime dust in such a manner that there is no longer any risk of recombination.
It may also be advantageous to take the air flow from the quantity of air preheated in the heat exchangers, since in this way a very hot mixture of Ca0 and air is obtained which allows substantial preheating of the combustion air to about 600.degree. C in a heat exchanger, whereby the consumption of fuel can be correspondingly reduced.
The objects aimed at by the invention could not be achieved if the burning process for obtaining CaO using the calorific value of the combustible constituents, more particularly the organic impurities of the sugar sludge, resulted in burning temperatures at which the CaO is over-heated or "dead burned". Therefore, it is particularly important and, in the exemplary process according to the present invention, possible in a relatively simple manner to conduct the calcining process for recovering CaO in such a manner that the organic constituents are completely burned, but over-heating and thus inadequate physical or chemical activity of the CaO are reliably obviated.
An important side object of the process according to the invention results from the production of CO.sub.2 which results from the burning of the organic constituents of the sludge or the additional fuel, and which is required in any case for the sugar production and thus can be economically used.
For the best possible economic conditions, raw material, i.e., the lime sludge before introduction into the chamber is preheated by means of exhaust gas heat to a temperature of about 400.degree. to 800.degree. C, preferably over 500.degree. to 600.degree. C. The combustion air is also preheated in an appropriate manner, and preferably to a temperature which is above the calcining temperature.
The sludge is conveniently blown into the heat treatment chamber (calcining chamber) and for this purpose it is advantageous to use a portion of the exhaust gas flow.
If in a sugar production works in which an installation for carrying out the process according to the invention as described hereinbefore is installed, at the beginning of a campaign no lime-containing sludge has been yet obtained from sugar production, or only an inadequate amount, or outside the campaign it is desired to produce burnt lime, the installation can be used without further modifications for the production of the required CaO by using as the starting material additionally to the sludge present or instead of the sludge, finaly ground limestone which is then subjected in the same manner as mentioned to a calcining treatment in order to obtain the desired CaO. If necessary, other calcinable materials with a suitable reaction may also be treated in a similar manner. From this there follows the possibility of using the installation in question also for the production of other graded, surface-active powders for obtaining in an advantageous rational manner suitable high-quality inorganic products.
Further novel features and other objects of this invention will become apparent from the following detailed description, discussion and the appended claims taken in conjunction with the accompanying drawings which disclose an installation for carrying out the process according to the invention for the recovery of lime from predried lime-containing waste sludges.